About Hydraulic Valves

Hydraulic valves contain and transfer the flow and pressure of hydraulic fluid in hydraulic power systems. They range from simple shutoff valves to precision control valves. Common types of hydraulic valves include angle, ball, block and bleed, check, control, cartridge, directional, drain, needle, poppet, pressure relief, safety, shut off, solenoid, spool, and stack mounted valves. In their unpowered state, hydraulic valves can be normally open (open center) or normally closed (closed center). Hydraulic valves with a tandem center connect the pressure and tank ports, but block the service ports to allow system unloading and isolation of the service lines. Devices with a float center allow the supply to be shut off and enable the load to move to other services.

There are many types of hydraulic valves. Angle valves admit media at an angle and permit maximum flow. Ball valves provide tight shut-off and reliable control. Block and bleed valves use a small port to depressurize the space between the inlet and outlet. Check valves prevent flow reversal. Control valves modify fluid flow. Directional valves steer process media through selected passages. Drain valves are used to remove surplus fluid from a system or container. Needle valves have a slender, tapered point at the end of a valve stem. Poppet valves open and close ports with a sealing device that includes a disk, cone, or sphere. Pressure relief valves remove excess upstream pressure. By contrast, regulators maintain a constant outlet pressure. Safety valves contain a thermal sensing component that opens or closes in response to temperature changes. Spool valves are actuated by a rotary or piston-like spool that slides back and forth to cover and uncover ports in the housing. Shut off valves close a line to stop flow when a pre-set condition occurs. Stack mounted, sandwich, or modular valves can be assembled to create a valve block.

Performance specifications for hydraulic valves include valve size, pressure rating, ports, media temperature, flow coefficient, and connection type. Valve sizes are typically measured in inches (in) while pressure ratings are usually measured in pounds per square inch (psi). Both double-port and multi-port hydraulic valves are available. Media temperature is a maximum amount expressed in degrees Fahrenheit. The flow coefficient equals the number of gallons of 60° F water that flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. There are several connection types for hydraulic valves. Some devices have internal or external threads for inlet or outlet connections. Others use a bolt flange, clamp flange, union connection, tube fitting, butt weld, or socket weld. With compression fittings, tightening a nut on one fitting compresses a washer around the other. With metal face seals, a gasket is sandwiched between two fittings.